1. Field of the Invention
The present invention grew out of the needs of the semiconductor industry for a sealed gas valve that has extremely low leakage and that does not introduce contaminants into the system in which it is installed. The gases whose flow is controlled by such valves are extremely pure and in some instances are aggressive, corrosive, toxic, or flammable.
The present invention employs a bellows that extends to close the valve and that contracts to open it, whereby the high purity gas flowing through the valve remains sealed from the ambient air.
2. The Prior Art
In earlier sealed valves, the seal was maintained by an O-ring, and the lowest leak rates were on the order of 10.sup.-7 standard cubic centimeters of gaseous helium per second, a leak rate that is unacceptably high for the application contemplated. In contrast, the valve of the present invention has a leak rate of less than 10.sup.-13 sccm/sec., the sensitivity of the test equipment.
One alternative to conventional O-rings that was explored was the possibility of replacing the O-ring seal by a weld. Welding was not a suitable alternative because the parts to be welded were located in a relatively inaccessible part of the valve, which made it impossible to remove the welding oxidation, which remained as a source of contamination. Also, the welded valve could not be disassembled for repairs or cleaning, so welding the parts was not an acceptable solution.
Thus, the present invention arose out of the need for a seal that could be opened and closed a number of times and that when closed would have an unprecedented low leak rate.